Networks of this description conventionally include a phase comparator with a first input receiving the incoming wave and with a second input receiving the locally generated oscillation, the output of this comparator controlling the local oscillator in a sense tending to reduce any phase difference existing between the two oscillations of substantially identical frequency fed to the comparator.
In steady-state operation, the output frequency of the local oscillator should faithfully follow any frequency drifts of the incoming wave but should not reflect any transient disturbances due, for example, to thermal noise or interfering signals. It is therefore desirable to insert a narrow-band low-pass filter between the comparator output and a control input of the oscillator so as to suppress high-frequency components of the control voltage. On the other hand, the presence of such a filter lengthens the acquisition period, i.e., the time required for establishment of the phase lock at the beginning of operations or upon temporary loss of signal.
A known and fairly obvious solution to the problem is a compromise on the bandwidth of the interposed filter to allow a reasonably fast acquisition at the expense of a certain irregularity in the local oscillation. Such a solution, of course, is not entirely satisfactory.
Another possible solution is a wobbling of the oscillator frequency to speed acquisition with a narrow-band filter. The presence of the wobbling signal, however, introduces complications since that signal must be cut off during steady-state operation and must be turned on again upon a temporary fading of the incoming wave. Similar complications are also involved with the use of a filter switchable between larger and smaller bandwidths.